[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

@AaronBallman @cor3ntin I believe we are getting close to finalizing this PR.
Would you be okay with this feature landing and myself approving this when it's ready?

There was some discussion here and in the RFC, but I don't think there was explicit approval (or objection) to land this, so I wanted to clarify this.

https://github.com/llvm/llvm-project/pull/102040